1. Field of the Invention
The present invention relates to a fixing device to fix an unfixed image formed on a recording medium as a fixed image onto the recording medium, and an image forming apparatus including such a fixing device.
2. Description of the Related Art
As a fixing device provided to image forming apparatuses such as copiers, printers, facsimile machines, and multifunctional machines including the above functions, there are those provided with a fixing belt stretched over three rollers as disclosed in JP-2007-108635-A and a fixing belt stretched over two rollers as disclosed in JP-2009-25464-A. Specifically, the former fixing belt is stretched over a fixing roller, a heat roller, and a tension roller. The latter fixing belt is stretched over the fixing roller and the heat roller without using the tension roller. Accordingly, the latter realizes greater compactness in the fixing belt. In the 2-roller arrangement, the heat roller also serves as a tension roller to give tension to the fixing belt.
FIG. 20 shows an example of a fixing device in which a fixing belt is stretched over two rollers.
The fixing device as illustrated in FIG. 20 includes a fixing roller 100, a heat roller 200 having a heat source 500 inside thereof, an endless fixing belt 300 stretched over the fixing roller 100 and the heat roller 200, and a pressure roller 400 provided opposite and in contact with the fixing belt 300 so that a nip is formed between the pressure roller 400 and the fixing belt 300.
When an image is fixed by this fixing device, first, the pressure roller 400 is driven to rotate by a driving motor, not shown, and the fixing belt 300, the fixing roller 100, and the heat roller 200 are driven by the driving of the pressure roller 400. Then, a sheet of a recording medium P on which an unfixed toner image T is carried is conveyed to the nip between the fixing belt 300 and the pressure roller 400 in the direction shown by the broken line arrow, and toner images T are fixed on the surface of the recording medium P with heat and pressure.
In addition, as illustrated in FIG. 20, the fixing device is in general provided with a temperature detector 600 such as a thermistor or a thermostat to detect a temperature of the fixing belt 300. Based on the temperature detected by the temperature detector 600, the fixing belt 300 is maintained at a predetermined temperature target value and is prevented from being heated excessively. The temperature detector 600, which may or may not contact the fixing belt, is provided opposite the heat roller 200 so that the temperature of the heated part of the fixing belt 300 may be detected easily. Further, to detect the temperature with higher precision, a fixed, constant distance between the non-contact temperature detector and the fixing belt, or a fixed, constant contact pressure between the contact temperature detector and the fixing belt are preferably retained.
However, in the fixing device of the type in which the fixing belt is stretched over two rollers as illustrated in FIG. 20, the heat roller 200 serves as a tension roller. Therefore, when the fixing belt 300 expands or shrinks due to changes in temperature, the heat roller 200 is designed to move toward or away from the fixing roller 100 in order to adjust the tension on the fixing belt 300. As a result, as illustrated in FIG. 21, if the heat roller 200 moves from its home position in the X- or Y-direction to positions indicated as 200X or 200Y, the distance between the temperature detector 600 and the fixing belt 300 changes also, and as a result the temperature cannot be detected precisely. The same outcome occurs if a contact-type temperature detector is used, in that the contact pressure between the temperature detector and the fixing belt changes due to the movement of the heat roller. In this case also, the temperature cannot be detected precisely. Moreover, when the fixing belt is rotated in a state in which the contact pressure between the temperature detector and the fixing belt increases, the surface of the fixing belt may be scratched by contact with the temperature detector.
In order to solve the above-described problem, JP-2000-81804-A discloses a technology in which the temperature detector is integrated into the heat roller via a support member. Accordingly, even though the heat roller moves, since the temperature detector moves integrally with the heat roller, the distance between the heat roller and the temperature detector remains constant. However, a support member to connect the temperature detector with the heat roller is needed. Thus, the number of parts and assembly steps increase, hindering efforts at more compactness or lower manufacturing cost cannot be realized.